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Old   November 21, 2018, 13:57
Default RCCI model ignition delay
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Gaurav Guleria
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Hi everyone,

I am working on validating dual fuel RCCI model using reformate and diesel as fuels. I have set up my spray using parameters from ECN Spray A provided in Converge studio which has been followed by other people in literature as well. The only change I have made is increasing the nozzle diameter. I am modeling diesel fuel as a surrogate mixture of n-dodecane and xylene (75:25 by mass) using the mechanism CSU186 by Hocket et al (2017). I am using this mixture as a surrogate of diesel in liquid as well as gas phase. But the pressure rise and heat release in my model is way earlier than the experiment. My SOI is -40 aTDC.

Can you recommend some parameters for the spray setup that I can change so that I can delay the evaporation of diesel and hence increase the ignition delay for the model?

I have attached the plots for the pressure and heat release rate for your reference
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File Type: jpg Pressure.JPG (85.2 KB, 35 views)
File Type: jpg HRR.JPG (77.6 KB, 30 views)
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Old   November 21, 2018, 14:21
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Sameera Wijeyakulasuriya
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Hello Gaurav,

Before tuning the combustion model, here are some of the things you should check. Often times, we blame the pressure mismatch on combustion model while the root cause might be residing some where else. We should try as much as possible to avoid getting the right result for the wrong reason. Only when we get the right result for the right reason, we are moving closer to predictive engine combustion modeling.

1. Are you simulating the intake flow with valve motion or starting the simulation at IVC with assumed thermodynamic and flow conditions? We recommend the former as the velocity and turbulence field is hard to assume accurately. The Tumble motion in the cylinder breaks down in to turbulence closer to TDC. accurate prediction of TKE around the ignition location is quite important for initial flame kernel growth.

2. How are you modeling "Reformate" fuel? What is the assumed surrogate? Do you know the Lower Heating Value (LHV) of the actual test fuel. Is the surrogate LHV matches the test fuel LHV? If not, we have to correct for that.

3. RCCI engine cases typically have large cycle to cycle variation in cylinder pressure. This means that the internal residual amount is changing from cycle to cycle. How do you account for residual gas amount and composition? If you are making assumptions here, I would recommend running multiple cycles to wash out the initial conditions.

4. Do you know the LHV of Diesel fuel used in the test engine? We have to correct the surrogate LHV to match the Diesel LHV

5. Did you check if the injection pressure is matched correctly? Early injection RCCIs have longer ignition delay and the fuel injected tend to penetrate towards the cylinder wall. Capturing fuel penetration correctly is important.

6. Note how your simulation pressure curve is always higher than the measured pressure curve, including the tail end. This is an indication that the energy content in the simulation is higher than that in the experiment. Uncertainties/errors in fuel mass and LHV can cause this.

Hope this helps,

Sameera
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Old   November 22, 2018, 13:21
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Hello Sameera,

I have tried to answer all your questions to the best of my abilities but in case you need more information about anything, feel free to contact me. Following were the questions you had posted in your reply to the original thread.

1. Are you simulating the intake flow with valve motion or starting the simulation at IVC with assumed thermodynamic and flow conditions? We recommend the former as the velocity and turbulence field is hard to assume accurately. The Tumble motion in the cylinder breaks down in to turbulence closer to TDC. TKE around the ignition location is quite important for initial flame kernel growth.

Ans: I am aware of the fact that assuming the velocity and the turbulence field leads to inaccuracies in the model. To avoid that I generally run 3 consecutive cycles starting from EVO so that turbulence field resolution is independent of the initial conditions.

2. How are you modeling "Reformate" fuel? What is the assumed surrogate? Do you know the Lower Heating Value (LHV) of the actual test fuel. Is the surrogate LHV matches the test fuel LHV? If not, we have to correct for that.

Ans. The reformate fuel is a multi-component fuel. It consists of species like C2H2, CH4, C2H4, CO, H2, CO2 and N2. If you want I can share the mass fraction of each component as well. But as you can see these fuel species are found in most of the chemical mechanisms available. And since this is the PFI injected fuel in the model, I just specify the mass fractions of each species according to the phi provided by my colleagues who run the experiments. I know the LHV for the test fuel but I don’t know how to check if the test fuel LHV matches the LHV of the species I use in the model. If you can briefly point out how to check that, it would be very helpful.

3. RCCI engine cases typically have large cycle to cycle variation in cylinder pressure. This means that the internal residual amount is changing from cycle to cycle. How do you account for residual gas amount and composition? If you are making assumptions here, I would recommend running multiple cycles to wash out the initial conditions.

Ans. I always perform multiple cycles to wash out the initial conditions.

4. Do you know the LHV of Diesel fuel used in the test engine? We have to correct the surrogate LHV to match the Diesel LHV

Ans. I know the LHV of the diesel fuel but I will need a little guidance in how to match the LHV for the surrogate mixture of n-dodecane and m-xylene.

5. Did you check if the injection pressure is matched correctly? Early injection RCCIs have longer ignition delay and the fuel injected tend to penetrate towards the cylinder wall. Capturing fuel penetration correctly is important.

Ans. I set up my spray model using the parameters from the ECN spray A setup in Converge studio. I wanted to avoid making any unnecessary changes to the spray model so I used the same values. On looking at the spray_rate_inj0.out I saw that the injection pressure is 10 Mpa (100 bar) and the experimental rail pressure is 700 bar. This difference between the pressure in model and experiment is really large. This might be happening because I am using a square rate shape but that’s because we don't have rate shape measurement apparatus in our lab. Can you advise me on what is the best course of action in this case?

Also, I am confused because even if the pressure of injector in my model is low, how is the diesel igniting earlier if the droplets are not breaking up. What are your thoughts on this?


6. Note how your simulation pressure curve is always higher that the measured pressure curve, including the tail end. This is an indication that the energy content in the simulation is higher than that in the experiment. Uncertainties/errors in fuel mass and LHV can cause this.

Ans. I looked at the experimental cumulative heat release per cycle which is coming out to be 670 J (ensemble average of 300 cycles) and for the model, if I look at the Integrated_HR in the thermo.out file it's coming out to be around 630 J. I agree with you that the uncertainities in the fuel quantities and LHV might be the reason for this.
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Old   July 23, 2021, 00:50
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Pradeep Raju
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Gaurav, did you get a solution for all your problems?
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